Reduction of nitrogen oxides (NOx) in gas turbines has been achieved by controlling combustion zone temperature and residence time, or the time a burning fuel spends in a temperature range at which NOx is efficiently produced. Premixing fuel with air to a mixture leaner than stoichiometric equivalence can reduce the peak combustion temperature. This strategy is called Dry Low NOx combustion (DLN), or sometimes Dry Low Emissions combustion (DLE), because it does not depend on water injection to reduce the combustion temperature. However, a lean burn that is optimum for energy efficiency and low emissions can be unstable in some conditions. To minimize the risk of flameout or incomplete combustion, many combustor designs use a non-premixed pilot flame to stabilize a primary premixed flame, thereby reducing combustion dynamics.
A pilot flame is normally a diffusion burn, which occurs when fuel is injected into the combustor without premixing it with air. The fuel diffuses in the combustor air and burns as it diffuses. The pilot flame front maintains a mixture in the diffusion where the fuel/air ratio is sufficiently rich to ensure combustion stability under nearly all operating conditions. However, a diffusion flame burns at a higher flame temperature and produces more NOx than does a lean premix burn. Combustion temperature and NOx production both peak at or near stoichiometric equivalence, where the amounts of oxygen and fuel are matched for the reaction. A relatively small pilot flame can produce most of the NOx in the combustor.
Peak flame temperature is also determined by the fuel selection. Increasing the carbon to hydrogen ratio of a fuel increases its flame temperature, and hence increases NOx production. Natural gas is a commonly used gas turbine fuel. It is a mixture of hydrocarbon gases, principally methane together with varying proportions of ethane, propane, butane, and other gases.
Dimethyl ether (CH3OCH3), or DME, can be produced from natural gas and it is one means of delivering natural gas from remote sources. Other means include liquefied natural gas and conversion of natural gas to products like methanol, diesel fuel, and jet fuels.